Nuclear magnetic resonance is a phenomenon exhibited by a select group of atomic nuclei and is based upon the existence of nuclear magnetic moments in these nuclei (termed "NMR active" nuclei). When an NMR active nucleus is placed in a strong, uniform and steady magnetic field, the spin magnetization of the nucleus precesses at a natural resonance frequency known as the Larmor frequency, which is characteristic of each nuclear type and is proportional to the applied field strength at the location of the nucleus. Typical NMR active nuclei include .sup.1 H (protons), .sup.13 C, .sup.19 F and .sup.31 P. The resonant frequencies of the nuclei can be observed by monitoring with an RF receiver the transverse magnetization which results after a strong RF pulse is applied at, or near, the Larmor frequency. It is common practice to convert the measured signal to a frequency spectrum by means of Fourier transformation.
In order to use the NMR phenomenon to obtain an image of a sample, a magnetic field is applied to the sample, along with a magnetic field gradient which depends on physical position so that the field strength at different sample locations differs. When a field gradient is introduced, as previously mentioned, since the Larmor frequency for a particular nuclear type is proportional to the applied field strength, the Larmor frequencies of the same nuclear type will vary across the sample and the frequency variance will depend on physical position. By suitably shaping the applied magnetic field and processing the resulting NMR signals for a single nuclear type, a nuclear spin density image of the sample can be measured. Because the NMR signal which is measured is a function of the total number of nuclei of a given type, it is common to use a nucleus which is found in abundance in the sample to be imaged. For example, .sup.1 H (protons) are commonly used because they are abundant in many materials and therefore, generate a large NMR signal.
The observed resonance frequency for a given nuclear type is the sum of the Larmor frequency and small frequency shifts due to a variety of factors. Consequently, a complex spectrum of frequencies is typically observed in the absence of a magnetic field gradient. Because the observed frequency dependence on nuclear position is the basis for forming an image, in order to resolve two spatially-distinct, but physically-adjacent position elements, the magnetic field gradient strength must be increased to a point where the resonance frequency of nuclei in each element are shifted from each by an amount which is greater than the natural spread in resonance frequencies.
Aside from a practical limit on the magnetic field gradient strength which can be generated with existing equipment, increasing the gradient strength also broadens the total spread of resonant frequencies over the entire sample width. As all of these frequencies must be accommodated by the RF receiver, the bandwidth of the receiver must also be increased. Noise enters the receiver in proportion to the square root of the receiver bandwidth so that as the magnetic gradient strength increases, additional redundant measurements must be taken to extract the signal from the noise. Since the redundant measurements require extra time, the amount of time needed to acquire an image therefore also places a practical limit on the image resolution. Consequently, most prior art techniques for increasing image resolution have attempted to reduce the resonance line width as much as possible rather than increasing the magnetic field gradient.
For spin=1/2 nuclei and for systems in which quadrupole effects can be neglected, the natural resonance line width in a sample is generally most influenced by three factors: dipolar couplings, chemical shifts and susceptibility broadening. Dipolar couplings result from interactions between the magnetic moments in neighboring nuclei. If the neighboring nuclei are of the same type, the perturbations are called homonuclear dipolar couplings and tend to broaden the characteristic resonance lines and reduce image resolution (for protons, homonuclear dipolar couplings are the most serious broadening factor). In liquids, the field perturbations induced by dipolar couplings are time averaged to zero by the continuous motion of individual molecules and thus do not severely affect image resolution, but in solids, these couplings can give rise to very large static magnetic field components which can be as much as several Gauss for interacting protons. A field perturbation of this magnitude can significantly widen a resonance peak and reduce image resolution.
Chemical shifts are also an important source of line broadening. More particularly, although identical nuclei have the same frequency dependence upon the magnetic field, differences in the chemical environment of each nucleus can modify the applied magnetic field in the local vicinity of the nucleus, so that nuclei in the same sample do not experience the same net magnetic field. The differences in the local magnetic field result in slight spectral shifts in the Larmor frequencies between two such chemically non-equivalent nuclei, called "chemical shifts" which tend to broaden the spread of resonance frequencies and reduce image resolution.
The chemical shifts have a component which is anisotropic in that it depends on the particular orientation of a molecule to the applied Zeeman field and an isotropic part which is independent of the applied field direction. In liquids, the rapid molecular motion tends to average out the anisotropic parts of the chemical shifts leaving the isotropic parts. However, in solids, the orientation of the solid molecules is relatively fixed with respect to the applied Zeeman field and, accordingly, the anisotropic chemical shift components do not average to zero, resulting in peak broadening.
Susceptibility broadening occurs because the bulk magnetic susceptibility of the sample varies across the sample. Variations in the bulk susceptibility disrupt the uniform magnetic field applied to the sample and create local fluctuations in the field strength. Since the Larmor frequency is proportional to the applied magnetic field strength, the local fluctuations cause line broadening when averaged over the entire sample.
Therefore, in solids imaging systems, it is important to suppress homonuclear dipolar couplings, chemical shifts and susceptibility broadenings (chemical shifts and susceptibility broadenings are referred to collectively as "inhomogeneous" broadenings) in order to obtain high resolution without increasing the magnetic field gradient.
Further, one of the most useful NMR parameters is the isotropic chemical shift which can be used to resolve two or more chemicals in a sample mixture. However, as mentioned above, in a solid, the isotropic chemical shift is generally obscured by the anisotropic chemical shift. For example, in a sample consisting of a mixture of paradimethoxy benzene and hexamethyl benzene, the resulting NMR output spectrum is approximately 50 Khz wide. This width can be reduced to a few parts per million using conventional line narrowing techniques, such as the MREV-8 radio frequency pulse sequence, to remove dipolar coupling effects. However, even with this conventional line width reduction, the spectral width of the paradimethoxy benzene (about 15 ppm) resulting from the anisotropic chemical shift is still wide enough to encompass and obscure the spectrum resulting from the hexamethyl benzene which is about 4 ppm wide but lies within the paradimethoxy benzene spectrum.
One prior art method of reducing some of the aforementioned broadenings and the anisotropic chemical shift consists of orienting the solid sample at the "magic angle" (54.degree.44') with respect to the applied Zeeman field and physically rotating the solid at a relatively rapid rate thereby causing the anisotropic parts of the perturbing components to average to zero. This technique is called "magic angle spinning" or MAS. In this case, for imaging purposes, the magnetic field gradient may also rotate in synchronism with the rotating sample so that the sample experiences a "constant field gradient". Since the isotropic chemical shift information is preserved, the MAS method allows many chemical mixtures to be resolved.
In the aforementioned example, MAS can reduce the spectral width of the paradimethoxy benzene spectrum to the point where it no longer encompasses the spectrum of the hexamethyl benzene so that the compounds can be resolved. It is then possible to use conventional techniques to select one line or a small group of spectral lines corresponding to one compound in a mixture of compounds in order to acquire an image. For example, either the spectral lines of paradimethoxy benzene or the spectral line of hexamethyl benzene of the previous example could be used to acquire an image to find the spectral properties of the sample mixture resulting from the fraction of the sample consisting of the selected compound.
In many imaging studies it is desirable to obtain an image from only a thin section or slice of an object in order to reduce the time needed to acquire the image data from the time needed for a full three dimensional image. Most NMR images are obtained as a two dimensional array of data points which are generated from a two dimensional slice of the test object. The thickness of the slice is usually determined prior to the imaging experiment either by physically cutting the object into a wafer or a slice and placing the physical slice into the NMR spectrometer or by placing the entire object into the spectrometer and relying on an NMR experiment to selectively excite or refocus a slice from the entire object. Typically, NMR slice selection is achieved by selectively exciting only a small region of the object in a region perpendicular to the plane of interest.
Physical slicing of the test object has obvious disadvantages in that the object is destroyed and the slice selection cannot be changed without physically changing the slice in the spectrometer. However, physical slicing is a simple technique which can be used with the aforementioned MAS line-narrowing techniques.
Prior attempts to apply NMR slice selection to imaging systems which use MAS line narrowing techniques have met with problems. Although a wide variety of slice selection techniques are known for both liquid samples and solid objects, the time variations in the slice selection magnetic field gradient used for MAS imaging makes these prior art techniques inappropriate for slice selection in MAS imaging.
One of the most straightforward approaches to MAS imaging is to create two magnetic field gradients which are both orthogonal to the spinning axis and orthogonal to each other. The field gradients are generated by means of known winding configurations called Golay coils. By applying quadrature sinusoidal currents to each set of gradient windings having frequencies which are the same as the spinning frequency, a magnetic field gradient is created which rotates with the sample. Consequently, when the magnetic fields are viewed from the sample, they appear to be static and the imaging experiment proceeds in a manner analogous to conventional non rotating imaging experiments.
For slice selection, a natural approach is to design the experiment to select slices perpendicular to the spinning axis. The most direct construction for generating the required slice selection gradient field along the spinner axis is a conventional coil geometry called a Maxwell pair which consists of two planar coils physically arranged parallel to each other on either side of the image plane of interest. The coils are located so that their axes are collinear and correspond to the center of the desired image. An image is then acquired in a plane parallel to the coil planes. The easiest way to apply such a coil arrangement to a MAS NMR system is to wrap the coils around the housing in which an MAS rotor containing the object spins. Consequently, the coils are oriented in such a manner that they are perpendicular to the spinning axis and are canted at the magic angle with respect to the main static magnetic field. The gradient coils are driven with a D.C. current so that, in the absence of rotation, the object experiences a linear gradient field along the rotor axis. However, the field strength is still dependent on the X and Y position.
Unfortunately, since the gradient coils are positioned at an angle to the main static magnetic field, as the sample rotates during the actual experiment, the main field adds or subtracts from the gradient field so that the overall magnetic field gradient in the static field direction is not constant but oscillates. By carefully designing the gradient coils, it is possible to produce a constant gradient along the rotor axis in the presence of the main static field, but the specially designed coils are no longer simple in construction.
In addition, the modulation of the dipolar couplings, chemical shifts and susceptibility shifts caused by the MAS technique also effectively sweeps the NMR resonance frequency through a relatively wide range of frequencies even in the presence of a constant magnetic field. Consequently, a slice selection scheme which depends on selective excitation of a small region of the object would, in fact, include a much larger region than is desirable due to the expanded resonance frequency range even in the presence of a constant magnetic field gradient.
Accordingly, it is an object of the present invention to provide a method for slice selection in an NMR solids imaging system.
It is another object of the present invention to provide a method for slice selection in an NMR solids imaging system which is suitable for use with the magic angle spinning technique.
It is yet another object of the present invention to provide a method for slice selection in an NMR MAS solids imaging system which does not use complicated gradient coil designs.
It is another object of the present invention to provide a method for slice selection in an NMR MAS solids imaging system which does not require physically slicing the test object.
It is still another object of the present invention to provide a method for slice selection in an NMR MAS solids imaging system by applying an RF pulse sequence to the object during an imaging experiment.
It is a further object of the present invention to provide a method for slice selection in an NMR MAS solids imaging system by utilizing a multiple pulse RF pulse sequence in conjunction with an oscillating magnetic field gradient.
It is yet a further object of the present invention to provide a method for slice selection in an NMR MAS solids imaging system in which a multiple pulse RF pulse sequence refocuses chemical shifts but imaging information is preserved by a time variant gradient field.